A graded rod lens having a refractive index that decrease parabolically from the center outward in a radial direction in becoming increasingly popular as an optical product for use in copiers, facsimile apparatus, and fiber optics. Such rod lens is made of either glass or plastic, and some such products have been used on a commercial scale (see Nikkei Electronics, Aug. 20, 1979, pp. 64-74). Among the commercial products, the Selfoc lens ("Selfoc" is a tradename of Japan Selfoc Co., Ltd.), produced from multicomponent glass by an ion exchange method, is well known.
The production of rod lens by the ion exchange method has an inherent limiting factor, viz., the velocity of diffusion of the modifier ion that changes the refractive index at temperatures that do not deform the glass. For example, because of the necessity of using a fast diffusing cation, only a monovalent ion such as Tl, Cs, Rb, K, Na or Li can be used as the modifier ion, and furthermore to reduce the uneven distribution of refractive index is not easy. A large difference in refractive index can only be achieved by using Tl, but Tl is so toxic that it is not easy to handle. Also, it takes an excessively long period of time to produce a rod lens more than 3 mm in diameter at temperatures necessary for ion exchange that do not deform or break the glass, so such lens cannot be manufactured on an industrial scale.
A new method that relies on an entirely different principle, referred to as "molecular stuffing", and which is free from the above defects, has more recently been developed. According to this method, a porous glass preform is prepared by phase separation, leaching, and washing; CsNO.sub.3 is solidified within the fine pores in the glass preform to provide a certain concentration gradient of CsNO.sub.3 ; and then the pores are collapsed so as to provide a glass rod doped with Cs.sub.2 O whose concentration decreases parabolically from the center outward. Details of this molecular stuffing method are described, e.g., in Japanese Patent Applications (OPI) Nos. 28339/75 which corresponds to U.S. Pat. Nos. 3,938,974 and 4,313,748, 12607/76 which corresponds to U.S. Pat. Nos. 4,110,093, 4,110,096, 4,220,682, 4,236,930, 4,313,748 and 102324/78 which corresponds to U.S. Pat. Nos. 4,188,198, 4,183,620 and 4,299,608 (OPI as used herein refers to an unexamined published Japanese patent application). However, for the very reason that the porous glass preform is prepared by phase separation, leaching, and washing, the open micropores produced in the leaching step have a defective pore size distribution (i.e., an annual ring or eccentricity), and as a result the desired variation in the distribution of the Cs.sub.2 O concentration (and therefore the desired distribution of refractive index) is not completely obtainable. Furthermore, the maximum porosity of the resulting porous glass preform is 50%, and a higher porosity can not be achieved industrially, so a glass rod prepared by "stuffing" using a concentrated aqueous solution of CsNO.sub.3, solidifying the CsNO.sub.3, unstuffing (leaching) the CsNO.sub.3, resolidifying the CsNO.sub.3 and collapsing the micropores has a difference between the minimum and maximum refractive index of about 3.0% at most. Therefore, a porous glass preform having open micropores of a uniform desired size distribution has been strongly desired.
Various types of method for manufacturing porous glass have been known. For example, porous glass can be produced by the so-called Vycor method, ("Vycol" is a trademark of Corning Glass Works), the zeolite method, the white carbon method, the colloidal silica method and the silica gel method. There are two variations of the silica gel process: in one variation, a mixture of an aqueous solution of sodium silicate with an acid is stirred vigorously at low temperatures to form a silica hydrosol, which is gelled, given a predetermined form, and leached; in the other variation, a mixture of a solution of silicate salt with an organic compound causing the Cannizzaro reaction is stirred under slight heating to cause a mild reaction until a gel is formed, and after the gel is given a predetermined shape, the gel is leached.
One object of this invention is to provide a process for producing a lens material, and particularly a rod lens material, having a desired distribution of refractive index, and particularly providing a desired large difference between the minimum and maximum refractive indexes, by using a porous glass preform having open micropores of a uniform size distribution.
Another object of this invention is to provide an inexpensive lens material of a desired size.